1. Field of the Invention
The present invention relates to a developing sleeve of a developing device of an electrophotographic image-forming apparatus such as a copying machine and a laser beam printer.
The present invention also relates to a developing apparatus employing the developing sleeve for developing an electrostatic latent image.
The present invention further relates to an image-forming apparatus employing the developing apparatus.
2. Related Background Art
The electrophotographic image-forming apparatus such as copying machines and laser beam printers visualizes an electrostatic latent image formed on an image-holding member by developing it as a toner image by means of a developing device.
Such a developing device generally employs a developing metal sleeve, and visualizes an electrostatic latent image on an image-holding member by delivering, with the developing sleeve, a developing agent from a developing-agent container to a developing section facing the image-holding member to visualize the electrostatic latent image as a toner image.
The developing agent includes one-component type of developing agents such as one-component magnetic toner and a one-component non-magnetic toner, and two-component type of developing agents employing a non-magnetic toner and a magnetic carrier. The material for the sleeve is selected to meet the properties of the developing agent.
For use of a magnetic toner, a magnetic force-generating means such as a magnet is provided inside the developing sleeve which is made of a non-magnetic metal. The surface of the sleeve is roughened so as to be suitable for retaining and delivering the toner and to cause sufficient frictional electrification. On the development, a developing bias voltage, which may be AC, DC, or superposition thereof, is applied to the developing sleeve to conduct satisfactory development. The developing sleeve therefor is made of an electroconductive metal.
The conventional developing sleeves have disadvantages below.
When the developing apparatus is driven at a high speed with a high drive frequency, the surface roughness, namely the fine projections and recesses formed on the surface of the sleeve, is gradually reduced in the course of a long-term of use, which will cause deterioration of the toner delivering performance and of the toner electrification performance of the sleeve to result in lowering of density of the copied image.
In a non-contact developing system in which the developing sleeve does not brought into contact with the image-holding member, thermal distortion of the developing sleeve gives adverse effects on the copied image. The thermal distortion causes the change of the distance between the developing sleeve and the image-holding member, namely an S-D distance, affecting the developing performance and the density of the copied image.
The disadvantages are reduced by using a less thermally deteriorating material or higher thermal conductive material as the base material of the developing sleeve. Generally, aluminum is used therefor in consideration of the cost. The aluminum, however, is less abrasion-resistant, and is not suitable for the developing sleeve for which high durability is required.
To offset the above disadvantages, the surface of the developing sleeve is usually coated with a suitable material. However, the roughness of the surface and the durability of the surface is difficult to simultaneously achieve without rise of production cost.
For instance, a ceramic material which is frequently used as the coating material has a high hardness and is less workable although it is superior in durability. Therefore, it is highly difficult to form a rough coating surface of the ceramic material. A thin and uniform coating layer of the ceramic is required for electrification of the toner, since the ceramic is an electric insulator. However, the thin and uniform film of the ceramic cannot be readily formed at a low production cost.